Liquid metals, in particular circulating liquid metals, may contain gas bubbles of more or less large size which are entrained by the liquid in all the parts of the industrial installation where these metals are used. The origin and the nature of these gases may be very diverse, but generally these gases disturb, sometimes in an extremely serious way, the operation of the installation.
In the case of fast neutron nuclear reactors cooled by liquid sodium, the gas contained in the sodium may be hydrogen coming from the steam generator in the case where this generator has a slight leakage, the cooling water being decomposed by the sodium.
This gas may also be argon or another inert gas which acts as an atmosphere covering the sodium and which may be driven by a circulation pump.
This gas may also be constituted by vapor bubbles of the liquid metal produced by the cavitation of a pump.
In the first case, a rapid detection of the hydrogen bubbles is necessary in order to detect a possible leakage in the steam generator which may require, if it is serious, the immediate stoppage of the reactor.
In the second case, it is also desirable to detect the argon bubbles in the cooling sodium of the reactor, since these argon bubbles are liable to reduce to a considerable extent the cooling capacity of the sodium and consequently to result in formation of hot points in the fuel assemblies which have an adverse affect on their behavior in service.
In the third case, these vapor bubbles formed by the cavitation of a pump will, by the implosion effect, result in erosions which adversely affect the life of the apparatus.
Methods are known for detecting gas bubbles, for example, in the case of hydrogen, the diffusion of this hydrogen through a wall in contact on one side with the liquid metal and on the other side with a medium at very low pressure. Such a method is, however, relatively complicated to employ and has a sensitivity which depends on the temperature of the sodium and, above all, a long response time.
Processes are also known for the ultrasonic detection or measurement of foreign bodies in a homogeneous medium. Ultrasounds have, for example, been used for measuring the quantity of vapor in the form of bubbles in water at high temperature and at high pressure contained in an enclosure. This process, however, cannot be transposed to the case of liquid metals, since it employs measurements of the velocity of ultrasounds and requires the presence of ultrasonic transmitters or receivers in the immediate vicinity of, or in contact with, the enclosure enclosing the liquid; this process indeed employs measurements of the time taken by ultrasonic waves to travel in water under pressure which may possibly contain vapor bubbles.
In the case of liquid metals, in particular in the case of liquid sodium for cooling a fast neutron nuclear reactor, the temperature of this liquid metal may reach temperatures much higher than 400.degree. C. which are transmitted to the wall of the enclosure surrounding the metal. Ultrasonic transmitter-receivers which are capable of operating correctly at these temperatures are very expensive and of doubtful reliability.